Electric flow-through liquid heaters, which are often described as electric tankless liquid heaters, heat liquids as they pass through the heat exchanger. The objective of such heaters is to heat liquid as it enters and passes through the heat exchanger to the desired set point by the time it is dispensed at the outlet of the heater. In concept, this process is relatively simple to achieve in closed loop systems in which the operating parameters for flow and temperature can be predetermined. Instantaneous and tankless hot liquid heaters are known in the art for the delivery of hot liquid at a point of use.
U.S. Pat. No. 3,909,588 issued to Walker et al. discloses an electric liquid heater using electrodes immersed in an electrically-insulated flow-through tank with controls sensing both liquid temperature and heating electrode current.
U.S. Pat. No. 4,337,388 issued to July discloses a rapid-response liquid heating and delivery system incorporating liquid heating means, liquid temperature sensing means, and proportional integral derivative (PID) method of closed loop control.
U.S. Pat. No. 4,638,147 issued to Dytch et al. discloses a microprocessor controlled flow-through liquid heater regulating heating power by switching combinations of heating elements of different wattages.
U.S. Pat. No. 4,829,159 issued to Braun et al. discloses a method of switching electrical heating elements loads to reduce switching transients by energizing all loads neither switched off nor full on in sequence.
U.S. Pat. No. 4,920,252 issued to Yoshino discloses a temperature control method for a plurality of heating elements by allocating a required actuating time within one cycle of a predetermined length of time.
U.S. Pat. No. 5,216,743 issued to Seitz discloses a thermoplastic heat exchanger for a flow-through instantaneous liquid heater including a control system using temperature comparisons.
U.S. Pat. No. 5,479,558 issued to White, Jr. et al. discloses a flow-through tankless liquid heater with a flow-responsive control means.
U.S. Pat. No. 5,504,306 issued to Russell et al. discloses a tankless liquid heater system incorporating a microprocessor based control sensing liquid outlet temperature, accepting an option remote temperature-setting means and providing control of heating elements by applying power in fractions of a power line cycle.
U.S. Pat. No. 5,866,880 issued to Seitz et al. discloses using a plurality of heating elements wherein each of the elements receives a substantially equal amount of power and the delay between each element being powered is no more than 32 half cycles.